The present invention relates to a zoom lens for forming an image on an imaging element such as a CCD sensor and a CMOS sensor.
In recent days, a zoom lens has been more frequently mounted on a small device such as a cellular phone, a portable information terminal, and an internet camera as well as a digital still camera for another additional value. In the zoom lens, a part of lenses or lens groups that compose a lens system moves along an optical axis thereof. Accordingly, it is possible to continuously change imaging magnification and successively increase and/or decrease an image of an object to various sizes.
In case of mounting the zoom lens onto a small-sized device, the whole length of the zoom lens is preferably as short as possible. However, since a zoom lens needs to have a configuration so as to move at least two of lens groups that compose the zoom lens upon changing magnification and focusing, it is necessary to secure a space within the zoom lens to move the lens groups therein. For this reason, it is difficult to attain miniaturization of the zoom lens.
Also in recent days, the number of pixels in an imaging element for capturing an image of an object as electrical signals has increased each year, and therefore the zoom lens has also been required to exhibit high performances such as satisfactory aberration correction performance and compatibility to high resolution.
Patent Reference describes a conventional zoom lens. The conventional zoom lens includes a first lens group that is composed of a lens having negative refractive power; a second lens group that is composed of two lenses, i.e., a positive and a negative lenses, so as to have negative refractive power as a whole; a third lens group having positive refractive power; and a fourth lens group having positive refractive power.
According to the zoom lens disclosed in Patent Reference, a composite focal length of the first lens group and the second lens group at a wide-angle end is limited within a certain range. Accordingly, it is possible to attain relatively satisfactory miniaturization in spite of a high magnification range, which is as high as three times.    Patent Reference Japanese Patent Publication No. 2001-343588
The zoom lens described in Patent Reference does not fully satisfy the demands for high performances and miniaturization, although it is possible to relatively satisfactorily correct aberrations with a small number of lenses.
Here, such demands for high performances and miniaturization are not demanded only in small-sized devices such as cellular phones. Even in devices such as digital still cameras for general users, there is the demand for changing a magnification of an image, especially changing an optical magnification with less image deterioration, whereas there is also a demand for a smaller thickness to enhance portability.
In view of the above-described problems, an object of the invention is to provide a small-sized zoom lens with high performances that can provide satisfactory high image quality.